Method for the continuous production of a gaseous hydrogen stream

ABSTRACT

A process for the production of a hydrogen gas stream having a CO content of less than 1 ppm having a production cycle comprising two phases: phase 1 includes a) purifying a synthesis gas in a PSA unit, b) recovering a hydrogen gas stream comprising a CO content of greater than 1 ppm, c) purifying the gas stream by adsorption in a TSA unit, and recovery a hydrogen gas stream exhibiting a CO content of less than 1 ppm, and phase 2 includes e) purifying the synthesis gas in a PSA unit, f) recovering a hydrogen gas stream having a CO content of less than 1 ppm, where throughout steps e) and f), the TSA unit is bypassed by the hydrogen gas stream and is regenerated.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International PCT Application No.PCT/FR2018/051786, filed Jul. 13, 2018, which claims priority to FrenchPatent Application No. 1757491, filed Aug. 3, 2017, the entire contentsof which are incorporated herein by reference.

BACKGROUND

The present invention relates to a process for the production, fromsynthesis gas, of a hydrogen gas stream exhibiting a CO content of lessthan 1 ppm.

The bulk production of hydrogen is conventionally carried out by thesteam reforming of a carbon-based source, such as natural gas, which isrich in methane (“SMR process”). This process results in the formationof a synthesis gas predominantly rich in H₂ and in CO. In order toobtain hydrogen, the synthesis gas will be post-treated, for example bypurification processes, indeed even distillation processes.

Several solutions are provided, depending on the degree of purityrequired and on the production cost targeted.

The production of ultrapure H₂ can be obtained by cryogenic distillationof a mixture rich in H₂/CO. Prior to the distillation phase, thesynthesis gas produced should be decarbonated and dehydrated. Thistechnique is advantageous when it is desired to produce the twomolecules H₂ and CO at the same time. The residue gas rich in H₂originating from a cold box for the cryogenic separation of CO containsapproximately 97%.

This same H₂/CO mixture (synthesis gas), originating directly from thesteam reforming furnace or which has been subjected to one or morestages of reaction with water (water-gas shift reaction), can also bepost-treated by passage over a PSA (Pressure Swing Adsorption) but, inthis case, due to the limits of the PSA and in order not to excessivelypenalize its output, the final product can contain impurities of N₂ orCO type at the level of a few ppm to several tens or hundreds of ppm.

For certain industrial applications (for example the feeding of a fuelcell), the critical molecule proves to be CO, which is a poison for thefuel cell if its concentration is greater than 200 ppb. There is a risk,in the future, of this value falling to 100 ppb in order to allow fuelcell technologies which are poorer in expensive (platinum type)catalyst.

One technique for purifying hydrogen comprising between 0.5 and 100 ppmof CO down to 100 ppb consists in passing the gas stream into anadsorbent (active carbon or molecular sieve) at very low temperature,typically that of liquid nitrogen. The disadvantage of this techniquelies mainly in the cost of the equipment (pressurized cryogenic tanks,heat exchangers, LN2 cryostats) and also the operating costs (LN2).

Another purification technique consists in passing the hydrogen througha palladium-covered metal membrane. As a result of the cost ofpalladium, this technique is also expensive. Furthermore, the hydrogenis recovered at low pressure as a result of the loss of head in themembrane.

Starting from this, a problem which is posed is that of providing animproved process for the production of a hydrogen stream devoid ofcarbon monoxide, without, however, resorting to cryogenic adsorption ormetal membranes.

SUMMARY

A process for the production of a hydrogen gas stream having a COcontent of less than 1 ppm having a production cycle comprising twophases: phase 1 includes a) purifying a synthesis gas in a PSA unit, b)recovering a hydrogen gas stream comprising a CO content of greater than1 ppm, c) purifying the gas stream by adsorption in a TSA unit, andrecovery a hydrogen gas stream exhibiting a CO content of less than 1ppm, and phase 2 includes e) purifying the synthesis gas in a PSA unit,f) recovering a hydrogen gas stream having a CO content of less than 1ppm, where throughout steps e) and f), the TSA unit is bypassed by thehydrogen gas stream and is regenerated.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 illustrates a schematic representation in accordance with oneembodiment of the present invention.

FIG. 2 illustrates the results of a TSA test fed with impure hydrogen inaccordance with one embodiment of the present invention.

FIG. 3 illustrates the results of breakthrough of the CO through a TSAwith different parametric conditions in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As illustrated in FIG. 1, one solution of the present invention is aprocess for the production of a hydrogen gas stream 5 exhibiting a COcontent of less than 1 ppm starting from synthesis gas 1, comprising aproduction cycle exhibiting two phases:

-   -   a first phase comprising the following successive stages:        -   a) a stage of purification of the synthesis gas 1 by            adsorption by means of a unit of PSA type 2,        -   b) a stage of recovery, at the outlet of the PSA unit, of a            hydrogen gas stream 3 exhibiting a CO content of greater            than 1 ppm,        -   c) a stage of purification of the gas stream recovered in            stage b) by adsorption by means of unit of TSA type 4 of            chemisorber type, and        -   d) a stage of recovery, at the outlet of the TSA unit, of a            hydrogen gas stream 5 exhibiting a content of less than 1            ppm,    -   a second phase comprising the following successive stages:        -   e) a stage of purification of the synthesis gas 1 by            adsorption by means of the unit of PSA type 2,        -   f) a stage of recovery, at the outlet of the PSA unit, of a            hydrogen gas stream 5 exhibiting a CO content of less than 1            ppm,            with, throughout stages e) and f), the TSA unit is bypassed            7 by the hydrogen gas stream and regenerated 6.

TSA is understood to mean Temperature Swing Adsorption processes.

In TSA processes, the adsorbent, at the end of use, is regenerated insitu, that is to say that the impurities captured are discharged inorder for said adsorbent to recover the greater part of its adsorptioncapabilities and to be able to recommence a purification cycle, theessential regeneration effect being due to a rise in temperature.

In processes of PSA type, the adsorbent, at the end of the productionphase, is regenerated by desorption of the impurities, which is obtainedby means of a fall in their partial pressure. This fall in pressure canbe obtained by a fall in the total pressure and/or by flushing with agas devoid of or containing little in the way of impurities.

As the case may be, the process according to the invention can exhibitone or more of the following characteristics:

-   -   during the second phase, the TSA unit is bypassed by means of a        bypass line 7,    -   the TSA unit comprises just one adsorber 4,    -   the adsorber comprises an active material which will chemisorb        at east a part of the CO,    -   the active material comprises nickel and/or copper,    -   the PSA unit 2 comprises at least two adsorbers which each        follow, in staggered fashion, a pressure cycle comprising the        adsorption, depressurization and repressurization stages,    -   the pressure cycle carried out during the second phase exhibits        a shorter duration than the duration of the pressure cycle        carried out during the first phase,    -   the pressure cycle during the first phase and the pressure cycle        during the second phase exhibit an identical duration and the        depressurization of the pressure cycle carried out during the        second phase is carried out at a lower pressure than the        pressure of the depressurization carried out during the first        phase. Specifically, in this case, the pressure will be lowered        by at least 50 mbar,    -   the PSA unit comprises at least one adsorber comprising a layer        of alumina, a layer of active carbon and a layer of molecular        sieve,    -   the duration of the first phase is greater than at least 10        times the duration of the second phase.

The present invention relates to the provision of ultrapure H₂, producedfrom a source of synthesis gas type or of gas very rich in H₂. Thismixture rich in H₂ will be purified the majority of the time in 2stages. The methodology provided combines two coupled subprocesses,namely:

-   -   first subprocess: the feed hydrogen gas stream is introduced        into a unit of PSA type comprising at least two adsorbers        charged with different adsorbents distributed in layers and        intended for the trapping of the impurities by physical        adsorption. The PSA will operate in short cycles (a few        minutes). The adsorbers of the PSA unit will follow, in        staggered fashion, the following successive stages: adsorption        at high pressure of the cycle, cocurrentwise decompression,        countercurrentwise decompression, elution, countercurrentwise        recompression, cocurrentwise recompression. Typically, the        adsorbers of the PSA units can contain a layer of alumina for        removing the water vapor, one or more layers of active carbon        for removing certain molecules, such as CO₂, CH₄ and the        majority of the CO and nitrogen, and one or more layers of        molecular sieve for forcing the removal of the traces of CO and        nitrogen. The hydrogen thus produced will have a        purity >99.999%; the residual impurities will be minimal (ppm to        a few tens of ppm). However, this hydrogen purity is not        sufficient for its use in a fuel cell as a result of the        presence of CO (content>ppm).    -   second subprocess: the impure hydrogen exiting from the PSA is        purified by passage through a TSA unit, which preferably        comprises a single adsorber charged with an active material        which will chemisorb the residual CO impurity. The TSA will        operate in long cycles: a few days, indeed even a week, indeed        even a month. The regeneration of the TSA will preferably be        carried out under a temperature of greater than 150° C. The        temperature of the regeneration stream (nitrogen, hydrogen or        mixture) will be raised by means of a heating system (reheater).        The duration of heating of the regeneration stream will be        maintained up to the end of the regeneration cycle; at the end,        the reheater will be halted, the reactor will be flushed and        rinsed with pure hydrogen, then isolated under an H₂ atmosphere.        The temperature of the TSA will return to ambient temperature by        convective exchange with its environment, or by circulation of        hydrogen at ambient temperature, or by circulation of liquid        coolant in the reactor. Once at ambient temperature, the TSA        will again be available and operational for a new purification        cycle.

A hydrogen gas stream exhibiting a CO content of less than 1 ppm isrecovered at the outlet of the TSA unit.

Throughout the duration of the regeneration of the TSA, the PSA isadjusted to “high purity”: in other words, the output of the PSA islowered and the production of hydrogen exhibiting a CO content of lessthan 1 ppm is rendered possible, either by reducing the time of theoverall cycle or, preferably, by reducing the depressurization pressure.

Moreover, throughout the duration of the regeneration of the TSA, thehydrogen gas stream is no longer purified in two stages but in a singlestage, and by means of the PSA unit. During this mode of operation, theTSA is then bypassed by means of the bypass line.

The solution according to the invention thus makes it possible tocontinuously produce a hydrogen gas stream exhibiting a CO content ofless than 1 ppm while limiting the amount of adsorbers of the TSA unitand while optimizing the output of the upstream PSA, which onlyexperiences a decline in its output in “high purity” mode for a fewhours per day, weeks, indeed even months.

Tests have shown the possibility of purifying, to a CO content of lessthan 0.1 ppm, a hydrogen stream contaminated by CO present in the formof traces (20-100 ppm). This purification by TSA is carried out atambient temperature and at the pressure delivered by the upstream PSA(typically in the range 15-35 atmospheres). The monitoring and theeffectiveness of the TSA purification are produced by means of acommercial analyser of FTIR (Fourier Transform InfraRed spectroscopy)type, the detection level for CO of which is < than 100 ppb. The TSApurification continues at least until the appearance (the breakthrough)of traces of CO at the adsorber outlet. The purification can continuebeyond in order to obtain the breakthrough curve of the CO as a functionof the purification time.

FIG. 2 illustrates the results of a TSA test fed with impure hydrogen(contaminated with 95 ppm of CO). The ultrapurification (<0.1 ppm) ismaintained for 30 hours or more; beyond, the CO breaks through. Theadsorption capacity of the desired chemisorbing material can thus becalculated by this type of test.

FIG. 3 also presents results of breakthrough of the CO through a TSAwith different parametric conditions, showing the impact of theoperating temperature. A temperature gradient of 10° C. significantlychanges the capture capacity of the chemisorbant.

This observation is important for the scaling of a TSA; the data takenat low temperatures will make possible TSA designs which areconservative, thus without risk, given that the ambient temperature(thus the operating temperature) will increase.

This ultrapurification brought about by the TSA is possible by means ofa material, for example, based on Ni, indeed even on Cu.

The adsorption capacity of Ni or of Cu for CO is very dependent on theoperating conditions.

Regeneration conditions (low pressure, that is to say of the order of orless than 1.5 bar, and temperature greater than 150° C.) have beendefined in order to use the adsorbent in TSA and the repeatability hasbeen confirmed. Depending on the operating conditions and the design,the TSA may, for example, be regenerated only once a month.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

The invention claimed is:
 1. A process for the production of a hydrogengas stream comprising a CO content of less than 1 ppm derived fromsynthesis gas feed gas, the method comprising a production cyclecomprising two phases: a first phase comprising the following successivesteps: a) purifying a synthesis gas feed by adsorption in a PSA unit, b)recovering, at the outlet of the PSA unit a hydrogen gas streamcomprising a CO content of greater than 1 ppm, c) purifying the gasstream recovered in step b) by adsorption in a TSA unit, and d) recoveryat the outlet of the TSA unit a hydrogen gas stream comprising a COcontent of less than 1 ppm, and a second phase comprising the followingsuccessive steps: e) purifying the synthesis gas by adsorption in thePSA unit, f) recovering at the outlet of the PSA unit a hydrogen gasstream comprising a CO content of less than 1 ppm, wherein, throughoutsteps e) and f), the TSA unit is bypassed by the hydrogen gas stream andis regenerated.
 2. The process as claimed in claim 1, wherein, duringthe second phase, the TSA unit is bypassed by means of a bypass line. 3.The process as claimed in claim 1, wherein the TSA unit comprises asingle adsorber.
 4. The process as claimed in claim 3, wherein thesingle adsorber comprises an active material which will chemisorb atleast a part of the CO.
 5. The process as claimed in claim 4, whereinthe active material comprises nickel and/or copper.
 6. The process asclaimed in claim 1, wherein the PSA unit comprises at least twoadsorbers which each follow, in staggered fashion during the firstphase, a pressure cycle comprising adsorption, depressurization andrepressurization stages.
 7. The process as claimed in claim 6, wherein apressure cycle carried out during the second phase exhibits a shorterduration than the duration of the pressure cycle carried out during thefirst phase.
 8. The process as claimed in claim 6, wherein: the pressurecycle during the first phase and a pressure cycle during the secondphase exhibit an identical duration and a depressurization of thepressure cycle carried out during the second phase is carried out at alower pressure than the pressure of the depressurization carried outduring the first phase.
 9. The process as claimed in claim 1, whereinthe PSA unit comprises at least one adsorber comprising a layer ofalumina, a layer of activated carbon and a layer of molecular sieve. 10.The process as claimed in claim 1, wherein the duration of the firstphase is greater than 10 times the duration of the second phase.